Gallium nitride (GaN)

Gallium nitride is a III-V semiconductor with a direct bandgap of 3.39 eV, corresponding to 366 nm in the near ultraviolet. Alloyed with indium, the gap narrows: InGaN quantum wells emit across the violet, blue and green, 405 to about 530 nm, and alloyed with aluminium it widens into the deep ultraviolet. Nearly every blue and green light-emitting device in use is built this way. A white LED is a blue InGaN chip, around 450 nm (2.76 eV per photon), coated with a phosphor that converts part of the blue into yellow.

The material resisted development for decades. GaN has no practical native substrate at low cost, so it is grown on sapphire or silicon carbide, with a lattice mismatch that leaves threading dislocation densities of order $10^8$ to $10^9$ cm⁻², densities that would ruin a GaAs or InP device. And p-type doping, with magnesium, did not work until the late 1980s, when Amano and Akasaki activated the magnesium acceptors with electron-beam irradiation and Nakamura then found that thermal annealing in a hydrogen-free atmosphere did the same. The first bright blue LEDs followed in the early 1990s, and violet InGaN laser diodes in 1996; the 2014 Nobel Prize in Physics was awarded to Akasaki, Amano and Nakamura for the blue LED. That InGaN emits efficiently despite the dislocations is attributed to localization of carriers at indium-rich regions of the well, which keeps them from reaching the defects.

GaN crystallizes in the hexagonal wurtzite structure, and its strong spontaneous and piezoelectric polarization creates built-in electric fields across InGaN quantum wells grown along the usual c-axis. The field pulls electrons and holes to opposite sides of the well, the quantum-confined Stark effect, reducing their overlap and the radiative rate; the effect grows with indium content and is one cause of the "green gap", the drop in efficiency between blue and red emitters. Growth on nonpolar and semipolar crystal planes reduces the fields. At high current density, InGaN LEDs also lose efficiency ("droop"), with Auger recombination the leading explanation.

GaN laser diodes supply the 405 nm light of Blu-ray drives; with an objective of numerical aperture 0.85 against a DVD's 650 nm and 0.60, the focused spot area is 0.19 times as large, which is where the capacity gain comes from. Blue GaN lasers at around 450 nm are used in projectors, in automotive headlamps, and at kilowatt powers for welding copper, which absorbs blue light far more strongly than infrared. The same wide bandgap and high breakdown field make GaN a leading material for high-frequency and power transistors. Emission energies for the alloys follow from the photon energy relation.

References: S. Nakamura, T. Mukai, M. Senoh, Appl. Phys. Lett. 64, 1687 (1994); S. Nakamura et al., Jpn. J. Appl. Phys. 35, L74 (1996); The Nobel Prize in Physics 2014, scientific background (Royal Swedish Academy of Sciences).